From U.S. patent description Ser. No. 5,745,368 there is known a method of voltage stability analysis in electric power systems. That description discloses a method which is appropriate for low and high voltage applications as well as differing types of loads and load variations. In that method, a nose point of a P-Q curve showing functional relation between voltage and power is found, from which the distances to points characterizing the reactive, active and apparent power are calculated, while a generalised curve fit is used to compute the equivalent or surrogate nose point. The determination of that point is achieved by approximating a stable branch and creating a voltage versus power curve, determining a plurality of stable equilibrium points on the voltage and load curve, using the plurality of determined stable equilibrium points to create and fit an approximate stable branch, calculating an approximate voltage collapse point and thereafter a voltage collapse index. The value of that index allows for predicting the occurrence of expected voltage collapse under specific conditions.
From a European patent application No. EP 1 134 867 there is known a method for the assessment of stability of electric power transmission networks. The method comprises the measurement of vector quantities for voltage and current in numerous points in the network, transfer of those data to the system protection centre, transfer of information regarding the operating condition of equipment in the substations of that network, and, on the basis of the acquired data, determination of at least one stability margin value of the transmission network. The measured vectors may be represented by quantities such as voltage, current, power, or energy connected with phase conductor or an electronic system.
The method for the identification of weak and/or strong nodes of an electric power system according to the invention can be possibly employed as a useful solution for the assessment of stability of power networks, for example, in the solution presented in the application EP 1 134 867, although the identification of weak nodes in networks can be made separately from the methods of network stability assessment as presented in the state of the art, and the method as such is not yet known.
On the other hand, from U.S. patent description Ser. No. 5,796,628 there is known a dynamic method for preventing voltage collapse in electric power systems. In the presented solution “weak areas” in networks are identified. These areas are defined as those parts of the network which do not withstand additional load. The solution introduced in that description consists in monitoring the power network through the surveillance of real-time data from the network, forecasting the near-term load of each branch of the network as well as the power demand in that branch on the basis of those data, and in order to estimate the system stability, such that each of the branches would be able to withstand the expected load, the amount of the margin of reactive and/or active load is defined. The proposed value of this load as well as the proposed voltage profiles are determined on the basis of the known power flow technique and the saddle-node bifurcation theory.